Produced zipapp is not reproducible due to `_bootstrap` file ordering

[timothyg-stripe]

👋 We are using shiv in reproducible mode, but we found that the output is not idempotent due to this block of code that adds the _bootstrap directory:

https://github.com/linkedin/shiv/blob/a353d10ecd785c6bd67ccd08b642437e7204be57/src/shiv/builder.py#L165-L177

In particular, iter_package_files(bootstrap) (which eventually calls Path.iterdir()) is not guaranteed to be in any particular order.